For a while I've been wondering why the moon looks so big on the horizon. I looked around the web and found 2 answers: 1) At the horizon the brain thinks that it's very far way compared to when it's above your head in the sky, but since they're really the same size in both places the brain corrects for this and assumes that the moon near the horizon is bigger. 2) When the moon is low down you have a lot of things on the ground to compare it with and it looks big relative to them. But when it's high in the sky you have nothing to compare it with. Is there any way to check which explanation is correct? And is it really just an illusion with no optical explanation? Thanks.